Chapter 27-Light

• Primary source of light is sun
• Secondary source or light is brightness of sky
• Other common sources: flames, white-hot filaments in lamps, glowing gases in glass tubes
• Everything made visible by the light it reflects
• Air, water or window glass allows light to pass straight through
• Thin paper or frosted glass allow passage of light diffused in directions so that we can’t see objects through them.

• 27.1 Early Concepts of Light

• Some of ancient Greek philosophers thought light consisted of tiny particles which could enter the eye to create vision.
• Socrates and Plato thought vision resulted from streamers or filaments emitted by eye making contact with an object.
• It was later found that light originated from external source and came to viewers eyes after interacting with other objects, which created vision.
• Some thought light traveled in waves, other thought it traveled in particles.
• Particle theory supported by fact that light seemed to move in straight lines instead of spreading out as waves do.
• Huygens provided evidence when light DOES spread like a wave (diffraction)
• Wave theory became the accepted theory in 19th century
• 1905 Einstein published theory explaining the photoelectric effect- light consists of particles later called photons (massless bundles of concentrated electromagnetic energy)
• Light has dual nature, part particle part wave

• 27.2 The Speed of Light

• Not known whether light travels instantaneous or w/ finite speed until latter part of 17th century
• 1675 Danish astronomer Olaus Roemer made first demonstration that light travels at finite speed by making careful measurements of Jupiter's moons, especially the innermost moon, Io.
• Huygens interpreted it as when the Earth is farther away from Jupiter, the moon seems like it's late because of the longer amount of time it takes the light. The moon passed into Jupiter's shadow at the predicted time, but the light took much longer to reach Roemer on earth.
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• http://light.physics.auth.gr/images/history/galileo/fizeau.jpg
• Speed of light= extra distance traveled/extra time measured
• 300 000 000/ 1000s= 300 000 km/s
• Most famous experiment measuring speed of light performed by Albert Michelson in 1880
• Light from intense source directed by lens to an octagonal mirror initially at rest. Mirror carefully adjusted so that beam of light reflected to stationary mirror located on mountain 35km away, then reflected back to octagonal mirror and into eye of observer.
• Distance the light had to travel to the distant mountain carefully surveyed, so Michelson had only to find the time it took to make a round trip. Accomplished by spinning the octagonal mirror at high rate
• Conducted experiments to find that the time for light to make a round trip was the same amount of time it took the octagonal mirror 1/8 of a rotation.
• He divided the 70 km round trip distance by the time. Michelson’s experimental value for speed of light was 299 920 km/s which we round to 300 000 km/s. Michelson received 1907 Nobel Prize in physics for experiment. He was the first American scientist to receive the prize.
• Speed of light in vacuum is a universal constant.
• Light so fast, it could make 7.5 trips around the earth in 1 second
• Light takes 8 minutes to travel from sun to earth and 4 years from next nearest star Alpha Centauri
• Light year-distance light travels in one year
• Alpha Centauri 4 light years away
• Our galaxy has diameter of 100 000 light years
• Some galaxies are 10 billion light years from Earth

• 27.3 Electromagnetic Waves

• Light is energy emitted by accelerating electric charges—often electrons in atoms.
• Electromagnetic Wave-partly electric and partly magnetic
• Radio waves, microwaves, and x rays all electromagnetic waves
• Electromagnetic Spectrum-range of electromagnetic waves
• Lowest frequency of light we can see is red (700 nm)
• Highest frequencies are nearly 2x the frequency of red and appear violet (roughly 400 nm)
• Infrared-electromagnetic waves of frequencies lower than red of visible light
• Ex: heat lamps
• Ultraviolet-Electromagnetic waves of frequencies higher than those of violet
• Responsible for sun burns

• 27.4 Light and Transparent Materials

• Light is energy carried in an electromagnetic wave that’s generated by vibrating electric charges.
• When light hits any piece of matter, the electrons in the matter vibrate
• The electron vibrations in the light transfer to the electrons in the matter.
• Similar to sound received by a receiver
• How receiving material responds when light hits it depends on frequency of light and natural frequency of electrons in material
• Visible light vibrates at very hight rate, more than 100 trillion times per second (10^14 hertz)
• If charged object responds to these ultrafast vibrations, must have very little inertia
• Transparent-materials that allow light to pass through in straight lines
• All materials that are elastic respond differently to different vibrations.
• Example :Bells and tuning forks have particular frequencies
• Natural vibration frequencies of electrons depend on how strongly they’re attached to nearby nucleus.
• Example: Electrons in glass have a natural vibration frequency in ultraviolet range
• Energy received by atoms can be passed onto neighboring atoms by collisions, causing the light to be reemitted
• If UV light interacts with an atom with the same natural frequency, the vibration amplitude becomes unusually large
• Atoms hang onto this energy for long time and makes many collisions w/ other atoms and gives up its energy in form of heat
• When electromagnetic wave has lower frequency than UV, electrons forced into vibration w/ smaller amplitudes.
• Atom holds energy for less time, w/ less chance of collision w/ neighboring atoms, and less energy transferred into heat
• Energy of vibrating electrons reemitted as transmitted light
• Glass is transparent to all frequencies of visible light
• Frequency of the reemitted light passed from atom to atom identical to that of the light that produced the vibration
• Time delays result in lower average speed of light through transparent material
• Light travels at different average speeds through different materials
• Light travels very slightly slower than speed of light in atmosphere, but usually rounded off to speed of light (c)
• In water, light travels at 75% off its speed in a vacuum or .75c
• In glass light travels around about .67c, depending on type of glass
• In diamond, light travels at .40c
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• http://micro.magnet.fsu.edu/primer/java/speedoflight/javalightspeedfigure1.jpg
• When light emerges from materials into air, travels at original the speed (c) again
• Infrared waves (frequencies lower than that of visible light) vibrate not only the electrons, but also entire structure (like glass)
• This vibration of the structure increases the internal energy of the glass and makes it warmer
• Transparent materials successively absorb and reemit light by the atoms it strikes

• 27.5 Opaque Materials

• Opaque-materials that absorb light without reemission and thus allowing no light through them
• Ex: wood, stone, people
• Any coordinated vibrations given by light to atoms and molecules turned into random kinetic energy—that is not internal energy. Materials become slightly warmer.
• In metals, outer electrons of atoms not bound to any particular atom. Free to wander with very little restraint thought material. That’s why metal conducts electricity and heat so well
• When light shines on metal and sets free electrons into vibration, energy does not ‘spring’ from atom to atom, but it’s reemitted as visible light which is seen as reflection which is why metals are shiney
• Our atmosphere transparent to visible light and some infrared but almost opaque to UVs

• http://www.theozonehole.com/images/uvrayjp.gif

• UVs responsible for sunburn
• Clouds semi transparent to UV which is why you can get sunburn on cloudy days
• UV reflects from sand and water which is why you can sometimes get sunburn while in shade of beach umbrella

• 27.6 Shadows

• Ray-thin beam of light
• Any beam of light is made up of bundle of rays
• When light shines on object, some rays are stopped while others pass on in straight line path
• Shadow- formed where light rays cannot reach
• Sharp shadows are produced by a small light source nearby or larger source farther away
• Usually a dark part on inside and lighter part around edges
• Umbra-total shadow (all rays blocked from reaching part of surface)
• Penumbra-partial shadow (Only some rays blocked from reaching part of surface)
• Penumbra appears where some of light is blocked, but other light fills in.
• Penumbra occurs where light from broad source only partially blocked

• Pg 412 figure 27.11
• Example: when moon passes between earth and sun during solar eclipse
• Because sun so large, rays taper to provide umbra and surrounding penumbra
• Moon’s shadow barely reaches earth
• Lunar eclipse-shadow from earth extends into space and moon passes into it
• Shadows will occur when light bent in passing through transparent material like water
• Light travels at different speeds in warm and cold water
• Difference bends light, just like layers of warm and cool air in night sky bend starlight and cause twinkling.

• 27.7 Polarization

• Light travels in waves
• Polarization-aligning of vibrations in transverse wave, usually by filtering out of waves of other directions (light caused to vibrate in one direction)
• Light traveling forward usually vibrates up and down, left and right, everything in between
• Single vibrating electron emits electromagnetic wave that’s polarized:
• Vertically vibrating electron emits light vertically polarized
• Horizontally vibrating electron emits light horizontally polarized
• Common light source, like incandescent, emits NOT polarized light because vibrating electrons that produce the light vibrate in random directions
• When the light from these sources shines on polarizing filter (like sunglasses) the light transmitted IS polarized
• Filter is said to have polarizations axis that is in direction of vibrations of polarized light wave
• All light will pass through pair of polarizing filters when polarization axis are aligned, but not when they’re perpendicular
• Pg 414 figure 27.17
• This behavior like filtering of vibrating rope that passes through pair of picket fences
• All of the vertical waves will pass, but none of the horizontal ones.

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• http://whatis.techtarget.com/WhatIs/images/polariza.gif
• Light that reflects at glancing angles from nonmetallic surfaces (glass, water, roads) vibrates mainly in plane of reflecting surfaces
• Glare from horizontal surface is horizontally polarized

27.8 Polarized Light and 3D Viewing

• Vision in 3D depends on the fact both eyes give impressions simultaneously each eye viewing a scene from slightly different angle
• Combo of views in eye-brain system gives depth
• Pair of photos/movie frames taken a short distance apart (about average eye spacing) can be seen in 3D when left eye sees only left view and right eye sees only right view
• Slide shows/movies accomplish this by projecting pair of views through polarization filters onto a screen
• Polarization axes are at right angles to eachother
• Overlapping pics look blurry to naked eye
• To see in 3D, viewer wears polarizing glasses w/ lens axes also at right angles.
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• http://www.3dglassesonline.com/how-do-3d-glasses-work/3-d-glasses-traditional.gif
• Each eye sees separate picture, just as in real life.
• Brain interprets 2 pictures as single picture w/ feeling of depth
• Depth also seen in compture-generated stereo-grams